I. Technical Field
The present invention relates to a fixed type constant velocity universal joint used in automobiles or various industrial machines.
II. Description of the Related Art
Fixed type constant velocity universal joints include joints of types such as a Rzeppa type (BJ) and an undercut-free type (UJ). As illustrated in FIG. 4, the fixed type constant velocity universal joint of the Rzeppa type (BJ) includes an outer ring 3 as an outer joint member having an inner surface 1 in which a plurality of track grooves 2 are equiangularly formed along an axial direction, an inner ring 6 as an inner joint member having an outer surface 4 in which a plurality of track grooves 5 paired with the track grooves 2 of the outer ring 3 are equiangularly formed in the axial direction, a plurality of balls 7 interposed between the track grooves 2 of the outer ring 3 and the track grooves 5 of the inner ring 6 so as to transmit torque, and a cage 8 interposed between the inner surface 1 of the outer ring 3 and the outer surface 4 of the inner ring 6 so as to retain the balls 7. In the cage 8, a plurality of pockets 9 for accommodating the balls 7 are arranged along a circumferential direction.
Further, a center of curvature O1 of the track grooves 2 of the outer ring 3 and a center of curvature O2 of the track grooves 5 of the inner ring 6 are offset with respect to a joint center O by equidistances F, F in opposite directions along the axial direction, respectively.
A shaft 10 is inserted into a center hole (inner diameter hole) 6a of the inner ring 6, and spline fitting is performed between the shaft 10 and the center hole 6a. The spline fitting allows torque to be transmitted therebetween. Note that a circular circlip 11 for preventing shaft detachment is attached onto an end portion of the shaft 10.
A hardened layer S is formed in each of the track grooves 2 of the outer ring 3. In the case of forming the hardened layers S, when the number of the balls 7 is six, there is a case that the hardened layers S are not formed, as illustrated in FIG. 4, at opening end portions (Japanese Examined Patent Publication No. Sho 63-5614). The hardened layers S are not formed for the purpose of increasing fatigue strength against tension stress which is repeatedly generated in opening end portions 2a of track-groove-bottom portions of the outer ring 3 when the fixed type constant velocity universal joint transmits a rotational force while forming, as illustrated in FIG. 5, a high angle θ max (maximum angle formed between a joint axis L and an axis L1 of the shaft 10).